The primary mammalian pacemaker, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, drives circadian rhythms in physiology and behavior that are synchronized with the environment. In temperate climates, animals use annual changes in day length, or photoperiod, to adjust their physiological state with changes in season. The way in which the clock encodes photoperiod is unknown, but two coupled oscillators, one synchronized to morning (M) and the other synchronized to evening (E), whose phase relationship varies with day length would allow the SCN to compute seasonal changes in photoperiod. These dual oscillators may be embedded in the molecular transcriptional and translational feedback loops that participate in endogenous circadian rhythm generation in the SCN (4). The clock gene, Period 1 (Perl), is an important component of rhythm generation and also participates in the synchronization of the clock to light. The current proposal will investigate role of the isolated SCN in encoding day length in photoperiodic Perl reporter rats generated by crossing the photoperiodic Fischer 344 (F344) strain with transgenic Per1::luciferase (Per1::luc) rats that express luciferase under the control of the Perl promoter. The anatomical locations of putative M and E oscillators will be investigated by imaging real-time Perl promoter activity in subdivisions of the SCN in photoperiodic rats exposed to either short or long day lengths. Singlecell luminescence imaging will be used to examine photoperiod-dependent differences in Perl promoter activity within an individual SCN neuron. This proposal will also investigate the role of the molecular clock in encoding the critical photoperiod permissive for reproduction by comparing the Perl promoter activity profile of rats exposed to either inhibitory or permissive day lengths for reproduction. Furthermore, it will be determined whether photoperiod encoding in the SCN differs between photoperiodic F344 and nonphotoperiodic Wistar rats. Seasonal variations in photoperiod may also affect neurological processes in humans. PUBLIC HEALTH RELEVANCE Many patients diagnosed with seasonal affective disorder (SAD) experience depressive episodes only during the fall and winter, and treatment with early morning bright light corrects the delay in their circadian rhythms. Therefore, investigating the role of the SCN in encoding photoperiod may lead to the development of novel therapeutic treatments for SAD patients.